Microorganisms are grown commercially in suspension cultures, and in solid-state and immobilized-cell fermentations. In immobilized-cell bioreactor systems, the substrate and microbial end-products are in a mobile liquid phase which is continually entering and exiting the reaction vessel. In this system, microbial cells are attached to or entrapped in an artifical or natural solid matrix, in an attempt to maintain a high population of slow or non-growing cells in the reaction medium. Some common matrix materials that are used for immobilizing microbial cells include calcium alginate and .kappa.-carrageenan.
A drawback of such immobilized cell fermentation systems is diffusion of substrate into and product out of the matrix. In addition, interfering enzymes must either be eliminated from the system or inactivated. Also, the substrate and end-products in immobilized cell fermentations must be of a low molecular weight. Furthermore, release of microorganisms from the support matrix may occur in the late stages of fermentation which, in turn, decreases the life of the system.
In contrast to immobilized cell reactor systems that entrap microorganisms within a matrix, biofilm formation is a natural form of cell immobilization. A biofilm develops when microorganisms attach to an inert support that is made of a material such as stone, metal or wood. Filamentous microorganisms and actinomycetes will naturally stick to solid surfaces. There are also non-filamentous bacteria that will produce an extracellular polysaccharide that acts as a natural glue to immobilize the cells. In nature, nonfilament-forming microorganisms will stick to the biofilm surface, locating within an area of the biofilm that provides an optimal growth environment (i.e., pH, dissolved oxygen, nutrients). Since nutrients tend to concentrate on solid surfaces, a microorganism saves energy through cell adhesion to a solid surface rather than by growing unattached and obtaining nutrients randomly from the medium.
Biofilm formation has been used in various industrial applications, as for example, biological oxidation or reduction of industrial wastes, "quick" vinegar processes, animal tissue culture, and bacterial leaching of ores. Various attempts have been made to develop solid supports for immobilizing microorganisms to form biofilm reactors. For example, corn cob granules, polypropylene sheets, supports made of a cellulosic material (i.e., cellulose acetate, polyester cloth, cotton flannel, rayon, wood pulp, polyethylenimine-coated cotton), and a polymer-reinforced cotton gauze have been developed for use as cell immobilizing supports.
A disadvantage of current solid supports is that there is relatively low cell growth on the supports, and the cells of the microorganism tend to slough off from the surface of the support during the fermentation process. Also, supports made solely from agricultural plant materials tend to clump up in the aqueous medium. In addition, with the present supports, it is difficult to control the thickness of the immobilized cell layer.
Therefore, an object of the invention is to provide a composition for making a solid support with an increased capacity for immobilizing microbial cells, and which will facilitate cell growth in a continuous fermentation system and help prevent cell wash-out from a reaction vessel in a continuous fermentation system.